1. Field of the Invention
The present invention relates to a magnetic bearing device for levitating an object in an arbitrary position out of contact with electromagnets under magnetic attractive or repulsive forces generated by the electromagnets.
2. Description of the Related Art
FIG. 1 of the accompanying drawings shows a conventional magnetic bearing device. As shown in FIG. 1, the conventional magnetic bearing device comprises a magnetizable object 105 to be levitated, a pair of electromagnets 101, 102 for generating magnetic forces to levitate and support the object 105, and a pair of positional displacement sensors 103, 104 for detecting a positional displacement of the object 105. The object 105 is disposed between the electromagnets 101, 102 and also between the positional displacement sensors 103, 104.
The object 105 is subject to electromagnetic attractive or repulsive forces generated by the electromagnets 101, 102. The electromagnetic attractive or repulsive forces generated by the electromagnets 101, 102 are compensated for by a control circuit based on a detected positional displacement signal produced by the positional displacement sensors 103, 104. The object 105 is levitated and supplied under the electromagnetic attractive or repulsive forces generated by the electromagnets 101, 102.
The control circuit 111 comprises a sensor circuit 110, a compensating circuit 108, and a driver 109. The sensor circuit 110 comprises an offset corrector 106 and a sensor gain adjuster 107.
The detected positional displacement signal produced by the positional displacement sensors 103, 104 is compared with a threshold level preset in the sensor circuit 110, and a differential signal is compensated for by the compensating circuit 108 to control the driver 109 to apply drive signals to the electromagnets 101, 102 so as to levitate and support the object 105 in a desired position between the electromagnets 101, 102.
The threshold level is preset so as to be substantially equal to the detected positional displacement signal produced by the positional displacement sensors 103, 104 when the object 105 is positioned at the center of a levitation range between the electromagnets 101, 102. The driver 109 energizes the electromagnets 101, 102 to levitate and support the object 105 so as to minimize the differential signal between the threshold level and the detected positional displacement signal.
In order to preset the threshold level, it has heretofore been customary to manually move the object 105 or move the object 105 under magnetic forces from the electromagnets 101, 102, in a mechanically movable maximum range between the electromagnets 101, 102. The offset corrector 106 calculates a middle value of the detected positional displacement signal from maximum and minimum values of the detected positional displacement signal which are produced by the positional displacement sensors 103, 104 when the object 105 is thus moved. The calculated middle value is used as the threshold level or zero point.
If the magnetic bearing device is very large or very long or if the object 105 to be levitated cannot be touched, then it is practically impossible to move the object 105 manually in the mechanically movable maximum range between the electromagnets 101, 102.
Adjusting the levitated position of the object 105 under magnetic forces produced by the electromagnets 101, 102 also needs special care. Specifically, if the positional displacement sensors 103, 104 have inaccurate output characteristics due to mechanical dimensional errors or individual variations of the positional displacement sensors 103, 104, or either the magnetic bearing mechanism or the control circuit 111 is replaced at the time of maintenance of the magnetic bearing device, then it is necessary to adjust the levitated position of the object 105 in the magnetic bearing mechanism with the control circuit 111 to be used in a new combination.
The magnetic bearing device has auxiliary supports (auxiliary bearings) for contacting and supporting the object 105 in case the object 105 is held at rest while being levitated or the levitation control process suffers a malfunction. If the auxiliary supports are damaged, broken or excessively worn to the extent that they fail to perform their own function, then such a failure cannot be detected while the magnetic bearing mechanism remains assembled.
It is therefore an object of the present invention to provide a magnetic bearing device which makes it easy to adjust a preset level for the levitated position of an object to be levitated and which is capable of easily detecting deformation or wear of an auxiliary support for the object to be levitated.
To achieve the above object, there is provided a magnetic bearing device comprising a pair of electromagnets, a magnetizable object to be levitated which is disposed between the electromagnets, a pair of positional displacement sensors for detecting a positional displacement of the object, and a control circuit responsive to a detected positional displacement signal from the positional displacement sensors, for outputting a control current to control magnetic attractive or repulsive forces produced by the electromagnets to levitate the object in an arbitrary position between the electromagnets out of contact therewith, the control circuit comprising offset correcting means for producing an offset-corrected signal from the detected positional displacement signal from the positional displacement sensors, position compensating means for compensating for the offset-corrected signal from the offset correcting means, a driver responsive to a compensated signal from the position compensating means for applying a drive signal to energize the electromagnets, control means for controlling the driver to successively energize the electromagnets to move the object in a mechanically movable maximum range between the electromagnets, detecting maximum and minimum values of the detected positional displacement signal from the positional displacement sensors, calculating a middle value between the maximum and minimum values, and comparing the middle value with a predetermined threshold level, and output adjusting means for adjusting the offset-corrected signal from the offset correcting means to substantially eliminate the difference between the middle value and the threshold level so that the middle value represents a position in which the object is to be levitated between the electromagnets.
The control circuit may further comprise means for selectively entering a manual or automatic setting process command to perform a process of controlling said control means to control said driver to successively energize said electromagnets to move said object in a mechanically movable maximum range between said electromagnets, detect maximum and minimum values of the detected positional displacement signal, calculate the middle value between said maximum and minimum values, and compare said middle value with a predetermined threshold level, and said output adjusting means to adjust the offset-corrected signal to substantially eliminate the difference between said middle value and said threshold level.
The magnetic bearing device may further comprise a pair of auxiliary supports for limiting a movable range of the object, and the control circuit may further comprise means for monitoring a change in the maximum and minimum values of the detected positional displacement signal when the object is moved in the mechanically movable maximum range between the electromagnets, thereby to detect when the auxiliary supports suffer a failure.
By successively energizing the electromagnets, the object is moved in the mechanically movable maximum range between the electromagnets. At this time, maximum and minimum values of the detected positional displacement signal from the positional displacement sensors are detected, and a middle value between the maximum and minimum values is calculated and compared with a threshold level to determine a position in which the object is to be levitated. An adjustment signal is applied to the offset correcting means to substantially eliminate the difference between the middle value and the threshold level. With this arrangement, it is possible to levitate and support the object in a desired position even if the magnetic bearing device is very large or very long or if the object to be levitated cannot be touched.
Even if the positional displacement sensors have inaccurate output characteristics due to mechanical dimensional errors or individual variations of the positional displacement sensors, or individual components of the magnetic bearing mechanism and the control circuit need to be replaced, the positional displacement sensors can be adjusted in a reduced period of time, and any burden imposed on the operator for such an adjustment is reduced.
It is also possible to detect when the auxiliary supports are damaged, broken or excessively worn based on a maximum change in the detected positional displacement signals while in the above process of determining the target levitated position of the object.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate a preferred embodiment of the present invention by way of example.